As for a signal processing circuit of a rotation detecting device, a technology described in, for example, a patent document 1, that is, JP-A-2006-234504 has been known in the past. The signal processing circuit of a rotation detecting device includes: first and second magnetic sensors that are disposed to be opposed to the periphery of a rotor, which rotates together with, for example, the tires of an automotive vehicle, in order to output rotational signals associated with the turning angles of the rotor, and that are formed with, for example, magnetoresistive elements (MREs); first and second waveform reshaping units that output the rotational signals, which are fetched from the first and second magnetic sensors, as first and second pulsating signals which have a phase difference of ¼; a rotating direction deciding unit that decides the rotating direction of the rotor on the basis of the first and second pulsating signals fetched from the first and second waveform reshaping units respectively, and outputs a direction deciding signal which represents the decided rotating direction; and a first output terminal through which the first pulsating signal outputted from the first waveform reshaping unit is outputted as it is. Moreover, the signal processing circuit of a rotation detecting device further includes an output deciding circuit that when the direction deciding signal representing the reverse rotation of the rotor is outputted from the rotating direction deciding unit, outputs the first pulsating signal, which is fetched from the first waveform reshaping unit, to a second output terminal without any change, and that when the direction deciding signal representing the normal rotation of the rotor is outputted from the rotating direction deciding unit, outputs a constant signal to the second output terminal in place of the first pulsating signal fetched from the first waveform reshaping unit. Consequently, when the rotating direction of the rotor is the direction of normal rotation, the first pulsating signal is outputted from the first waveform reshaping unit to the first output terminal, and the constant signal is outputted from the output deciding circuit to the second output terminal. On the other hand, when the rotating direction of the rotor is the direction of reverse rotation, the first pulsating signal is outputted from the first waveform reshaping unit to the first and second output terminals. Thus, rotational information including the rotating direction of the rotor is produced based on a combination of kinds of signals to be outputted to the first and second output terminals. The rotational information is outputted to, for example, an electronic control unit (ECU) for vehicle control which is connected in a stage succeeding the signal processing circuit of a rotation detecting device.
The technology described in the patent document 1 does not take measures against noise that is superposed on rotational signals to be transferred between the first and second magnetic sensors and the first and second waveform reshaping circuits. Consequently, when first and second rotational signals are transmitted from the first and second magnetic sensors to the first and second waveform reshaping circuits, if noise is superposed on the first and second rotational signals, the rotating direction deciding circuit decides the rotating direction of the rotor on the basis of the first and second pulsating signals that are produced by reshaping the first and second rotational signals on which the noise is superposed. Consequently, there is a fear that the precision in decision on the rotating direction of the rotor may be degraded due to the noise superposed on the rotational signals.
Interposition of a noise removing circuit described in, for example, a patent document 2, that is, JP-A-2000-134070 between each of the first and second waveform reshaping circuits and the rotating direction deciding unit is conceivable. Incidentally, the noise removing circuit described in the patent document 2 includes: eight D flop-flops that sequentially delays by a predetermined time digital input signals (herein first and second pulsating signals outputted from the first and second waveform reshaping circuits); an AND circuit that outputs an output signal equivalent to the AND of the output signals of the eight D flip-flops; a NOR circuit that outputs an output signal equivalent to the negative OR of the output signals of the eight D flop-flops; and an SR flop-flop having the output terminal of the AND circuit and the output terminal of the NOR circuit connected to the reset terminal and set terminal thereof respectively. Consequently, the adverse effect of the noise, which is superposed on the first and second rotational signals, on the precision in the decision performed on the rotating direction of the rotor by the rotating direction deciding circuit can be minimized.
However, when the foregoing technologies described in the patent documents 1 and 2 are used in combination, a problem described below may arise.
Specifically, since the D flop-flops (delayers) are employed in the noise removing circuit described in the patent document 2, the phases of the rotational signals having the noises thereof removed by the noise removing circuit lag by a predetermined time behind the phases of the rotational signals outputted from the first and second waveform reshaping circuits. The predetermined time depends on the delay time offered by the delayers.
Herein, assume that before the predetermined time elapses with a time point, at which the signal level of the first pulsating signal outputted from the first waveform reshaping circuit is changed from a signal level associated with a logical high state to a signal level associated with a logical low state, regarded as an initial point, the adverse effect of the noise superposed on the first rotational signal is manifested and the signal level of the first pulsating signal is changed to the signal level associated with the logical high state. At this time, the noise shall also be superposed on the second rotational signal but the adverse effect of the noise shall not be manifested in the second pulsating signal. Incidentally, this situation does not take place under special circumstances. Since predetermined thresholds specified in the first and second waveform reshaping circuits are used to reshape the waveforms of the first and second rotational signals, which are analog signals, into the waveforms of the first and second pulsating signals, it is a matter of commonplace that the adverse effect of the noise is manifested in only one of the pulsating signals.
In the foregoing situation, the phase of a first filtered signal having the noise thereof removed by a noise removing circuit connected in a stage succeeding the first waveform reshaping circuit is delayed for a long period of time to lag behind the phase of a second filtered signal having the noise thereof removed by a noise removing circuit connected in a stage succeeding the second waveform reshaping circuit. Consequently, the sequence of the changes to logic levels occurring in the respective signals is reversed. Since the rotating direction of the rotor is decided based on the phase relationship between the first and second filtered signals, if the sequence of the changes of logical level is reversed, the rotating direction of the rotor may be incorrectly decided.
Thus, it is required to provide a signal processing circuit of a rotation detecting device capable of producing and outputting accurate rotational information that includes the rotating direction of a rotor which rotates along with the rotation of an object of detection.
Also known is, for example, a technology described in a patent document 3 that is JP-A-2007-170922 corresponding to US Patent Application Publication No. 2007/0139036. According to generally known technologies including the technology described in the literature, a signal processing circuit of a rotation detecting device includes: first and second magnetic sensors that are disposed to be opposed to, for example, the periphery of a crank rotor (rotor) which rotates along with the rotation of a crankshaft (object of detection) of an onboard engine, and that output rotational signals dependent on the rotation of the crank rotor; first and second waveform reshaping units that fetch first and second rotational signals outputted from the first and second magnetic sensors, reshape the waveforms of the first and second rotational signals, and output first and second pulsating signals which have a phase difference; and a reverse rotation deciding unit that fetches the first and second pulsating signals outputted from the first and second waveform reshaping units, and decides reversal of the rotating direction of the crank rotor on the basis of the relationship between the phases of the first and second pulsating signals. Moreover, the signal processing circuit of a rotation detecting device includes: a mask unit that masks one pulse part of the first pulsating signal which ranges from the first rise of the first pulsating signal to the first fall thereof and which occurs immediately after the reversal of the rotating direction of the crank rotor is decided by the reversal rotation deciding unit, and produces and outputs a masked signal which is a signal having the logical level of the masked first pulsating signal reversed; a rotating direction deciding unit that decides the rotating direction of the crank rotor on the basis of the relationship between the phases of the first and second pulsating signals outputted from the first and second waveform reshaping units; and an output unit that outputs the masked signal, which is fetched from the mask unit, as an output signal within a signal level band which differs from one to another according to the result of the decision concerning the rotating direction of the crank rotor performed by the rotating direction deciding unit. Consequently, rotational information including the result of the decision concerning the rotating direction of the crank rotor is produced, and the output signal is outputted to, for example, an ECU for vehicle control which is connected in a stage succeeding the signal processing circuit of a rotation detecting device.
If the technology described in the patent document 1 is applied as it is to, for example, a crank rotor that rotates along with the rotation of a crankshaft, problems described below arise. Specifically, for example, when a vehicle in which the signal processing circuit of a rotation detecting device is mounted is stopped, the rotation of the crankshaft is ceased. Consequently, the rotation of the crank rotor is ceased. Therefore, an output signal of the signal processing circuit of a rotation detecting device cannot have the waveform changed. However, a microscopic vibration may occur in the crank rotor due mainly to a backlash between the crankshaft and crank rotor and a vibration of the vehicle. When the microscopic vibration occurs in the crank rotor, although the crank rotor is substantially stopped, the rotating direction deciding unit included in the signal processing circuit of a rotation detecting device may incorrectly decide that the rotating direction of the crank rotor is switched to the direction of normal rotation or the direction of reverse rotation for a short period of time. In the technology described in the patent document 1, the result of the decision performed on the rotating direction of the rotor by the rotating direction deciding unit is used to immediately change the combination of kinds of signals to be outputted to the first and second output terminals. There is a fear that an ECU for vehicle control connected in a succeeding stage cannot appropriately execute various vehicle controls.
In the general technologies including the technology described in the patent document 3, an output signal is not produced by utilizing the first pulsating signal as it is, but the output signal is produced by utilizing a masked signal having one pulse part of the first pulsating signal, which ranges from the first rise thereof to the first fall thereof and occurs immediately after the reversal of the rotating direction of the crank rotor is decided. The output signal is immediately outputted to the ECU for vehicle control connected in the succeeding stage. Consequently, if a vehicle vibrates, the number of pulses contained in the output signal would decrease. Therefore, the ECU for vehicle control connected in the succeeding stage can decide that the rotating speed of the crank rotor is nearly null. In other words, it is possible to decide that the vehicle is substantially stopped. However, although the crank rotor itself is substantially stopped, it is still decided that the rotating direction of the crank rotor is frequently switched to the direction of normal rotation or the direction of reverse direction for a short period of time. Based on the result of the decision, the output signal is produced within a different signal level band, and outputted to the ECU for vehicle control connected in the succeeding stage. Consequently, the fear that the ECU for vehicle control connected in the succeeding stage may not be able to appropriately execute various vehicle controls cannot be swept aside.
Thus, it is required to provide a signal processing circuit of a rotation detecting device capable of producing and outputting accurate rotational information including the substantial rotating direction of an object of detection.